Carbonizing subdivided solids



Filed Sept. 19, 1947 CASE O0 .5 PRODUCT OUTLET 6 5 f 7 a a I E z m 5 w s4 o 2 Am fill. L A O E I c c r z w Y n Do 7 c 3 m 4 m o 2/ sww m .fi flv. www. w v q l WM Ju n 1. f Kw m. UH WW m. 3 M n mwwwmw H U 5m 7ACTOR lr/ms FEED LINE- 72 u 122 /z/NG GAS l/VLET' atented Got. 14*, 1952 72,614,069 CARBONIZING sonmvmzn soups 7 George L. Matheson, Union, N. 5.,assignor to Standard Oil Development Company, a corporation of DelawareApplicationseptember 19, 1947, Serial No. 774,995

11 Claims.

The present invention relates to the treatment of subdivided solids.More particularly, the invention pertains to a process of contactingsubdivided solids with upfiowing gases in an enlarged contacting chamberat a controlled gas flow rate adapted to maintain the subdivided solidsin the form of a highly turbulent ebullient mass resembling a boilingliquid.

Prior to the present invention, subdivided solids have been contactedwith gases by passing the latter upwardly through an enlarged contactingzone containing a body of the subdivided solids and controlling thesuperficial gas Velocity in such a manner as to maintain the solids in aquasi-liquid or fluidized state within the contacting zone. Thisquaisi-liquid fluidized state involves arapid circulation of thesubdivided solids in all conceivable directions throughout the fluidizedbed.

7 The advantages of processes using this type of fluidized solids aregreat in number and importance. For example, the contact between gasesand solids, as well as between individual solid'particle'sfisconsiderably improved as comparedwith other types of operation. Asubstantially uniform temperature may be maintained throughout afluidized bed of subdivided solids because of the rapid circulation andhigh turbulence of the solids within the bed which result in anextremely efiicient transfer of heat from particle to particle andbetween different sections of the bed. For the same reasons heat may beadded to, or extracted from a fluidized solids bed with the greatest ofease and speed. I

fluid operations of the character described above havebe'en employed formany processes included reduction and oxidation reactions,polymerization processes, the carbonization or gasification ofcarbonaceous solids, such as coal or the like, and a large number oiother exothermic and endothermic reactions. More specifically,successful use of the fluid solids technique has been made in variouspetroleum oil refining processes, such s catalytic cracking, reforming,hydrogenating and similar operations, as well as in the catalyticsynthesis of hydrocarbons from carbon monoxide and hydrogen. However,while the application of subdivided solids in the form of fluidized bedshas found extensive uses, there are certain inherent limitations in thistechnique which have prevented its adaptation in some fields and limitedits efiiciency in others. One of the more serious limitations of thefluid solids technique results from the fact that proper iluidization isbound to a definite particle size or 2 particle size distribution forany given superficial velocity or range of superficial velocity or thefluidizing gas. For example, some materials may be properly fluidized ata superficial gas velocity of, say, about 0.1 to 3 feet per second and aparticle size distribution ranging from about 30 to 200 microns.Particlesconsiderably larger than the size range indicated will tend tosettle out of the fluidized bed and particles considerably smaller thanthe indicated range will be carried to and beyond the top of the bed-bythe fluidizing gas, thus, destroying the particle size distributiondesirable for proper fluidization at the prevailing gas velocity.Neither the unduly large settled particles nor the unduly smallentrained particles may derive the full benefit from the advantageouscharacteristics or the fluidized bed. This situation becomesparticularly troublesome when it is desired to form fluidized solidsbeds of materials which are naturally occurring or artificially producedin the form of subdivided particles whose size spreads over a widerange.

For example, many types of waste'coal are obtained in the course ofconventional coal mining processes in the form of masses composed ofparticles varying in size from a few micronsfto as much as 5, 10 or moremillimeters. When masses of this type are treated in fiuidcarbonizationor gasification units with fluidizing gases 'or gasifying media, such assteam, carbon dioxide, oxygen-containing gases, 'or the like, the'supe"rficial elocity of the fluidizing gas is usually controlled atabout 1.0 to 3 feet per second at which proper fluidization of the bulkof the coal particles having particle size distributions within theapproximate range of from about 1 6 'to 5 millimeters 'may beaccomplished. However, under these conditions coal particles having -aparticle size of substantially more than 5 millimeters, say, up to about10 to 15 millimeters settle out of the fluidized bed and coal particlesconsiderably smaller than M of a millimeter, say, of about 10 to micronsare blown out of the fluidized bed by the rluidizing gas. In addition,when fluidization is continued over a substantial length of time, atendency develops toward a separation of particles having sizes Withinthe upper originally fluidizable brackets of, say, about 2 to 5millimeters from particles having sizes within the lower originallyfluidizable brackets of about to 1 millimeter. Asa result, substantialproportions of the coal feed, which may'amount to as much as 10-30%,arelost either completely or to the desired treatment at optimumconditions. In addition, the superficial gas velocity of the fluidizinggas must be continuously checked and readjusted in order to compensatefor the continuous shift in particle size distribution.

Similar difficulties arise in various catalytic processes involving asignificant change of a catalyst particle size during the catalyticreaction. An outstanding example for such processes is the synthesis ofhydrocarbons from carbon monoxide and hydrogen employing fluidizedirontype catalysts at temperatures of about 500- 700" F. and pressuresof about EEO-50 atmospheres. It is well known that iron-type synthesiscatalysts at these conditions have a strong tendency to carbonize, thatis to form catalyst deposits of free carbon or coke-like materials. Influid operation, carbonization leads to a rapid disintegration of thecatalyst resulting in an equally rapid expansion and the ultimate lossof the catalyst bed in the form of catalyst fines entrained in thegaseous reaction products. It has been suggested to alleviate thesedifficulties by continuously or intermittently feedin fresh orregenerated coarse catalyst to the reaction zone in order to establish aparticle size distribution desirable for proper fluidization. However,the above mentioned tendency of the particles of different size toclassify, is'not avoided in this manner.

The present invention overcomes the aforementioned difiiculties andaifords various additional advantages. These advantages, the nature ofthe invention and the manner in which it is carried out will be fullyunderstood from the following description thereof read with reference tothe accompanying drawing.

It is the principal object of the present invention to provide improved.means for maintaining proper fluidization conditions within a fluidizedbed of subdivided solids having particle sizes spreading over a widerange.

A more specific object of the present invention is to provide means formaintaining a desirable particle size distribution Within a fluidizedbed of subdivided solids having particle sizes spread over a wide range.

Other and more specific objects and advantages of the invention willappear hereinafter.

In accordance with the present invention, these objects and advantagesmay be accomplished, quite generally, by supplying to a fluidized bed ofsubdivided solids comprising com ponent particles having a tendency tomove in one vertical direction at the fluidization conditions, suchparticles as have a tendency to move in an opposite direction andselecting the point of this particles supply and substantially removedfrom the center of the fluidized bed in the direction of the motion ofsaid component particles. In other words, the invention provides forsupplying to a fluidized bed of solids comprising particles large enoughto sink to the bottom of the fluidized bed at the prevailin fluidizationconditions and/or particles small enough to rise at least to the top ofthe fluidized bed at the prevailing fluidization conditions, a fractionconsisting preponderantly of particles of said large size to an upperportion of the fluidized bed and/or a fraction consisting preponderantlyof particles of said small size to a lower portion of the fluidized bed.By this means, the small particles tending to rise upwardly through thefluidized bed encounter an excess of coarse particles and the largeparticles tending to sink to the bottom of the fluidized bed encounteran excess of small particles in their respective directions ofclassification so as to establish the particle size distributionadequate for proper fluidization within the center as well as within thetop and bottom portions of the fluidized mass. In this manner,classification of particles varying greatly in size within the fluidizedbed as well as an undesired removal of small and large particles fromthe fluidized bed may be substantially reduced or completely eliminated.

More specifically, in processes involving the continuous or intermittentfeed to a fluidized solids bed, of a subdivided solids charge varying inparticle size over a Wide range, the charge, in accordance with thepresent invention, is divided into at least 2 fractions which differgreatly in average particle size. A fraction comprising predominantlyparticles of the lowest size ranges is fed to a bottom portion of thefluidized bed While a fraction comprising predominantly particles of thehighest size ranges is fed to an upper portion of the fluidized bed.Fractions composed predominantly of particles of intermediate size orhaving substantially the composition of the unfractionated chargematerial may be fed to intermediate sections of the bed, if desired.This procedure is particularly suitable for the treatment ofcarbonaceous solids such as coal, lignite, peat, oil shale, tar sands,coke, oil coke, cellulosic materials including lignin, etc., which areobtained from natural or artificial sources or specifically prepared forthe purpose in the form of subdivided masses, the particle size of whichspreads over wide ranges.

When applied to processes involving a change of particle size during thetreatment of solids in a fluidized bed, the objects of the invention maybe accomplished by feeding the solid undergoing treatment either in arelatively small particle size to a lower portion of the fluid solidsbed 01' in a relatively large particle size to an upper portion of thefluid solids bed depending on whether the change of particle size takingplace within the bed is of the type of disintegration or enlargement. Anexample for this application of the invention is the catalytic synthesisof hydrocarbon from carbon monoxide and hydrogen over iron catalysts,mentioned above.

, When catalyst disintegration begins adversely to affect fluidizationconditions, fresh or regenerated catalyst of a particle size,substantially larger than the fines formed by disintegration, is chargedto the upper portions of the fluidized bed in proportions adequate tomaintain a suitable particle size distribution throughout the bed.

It will be appreciated that the absolute and relative amounts ofsubdivided particles of relatively large and relatively small particlesize to be supplied to fluidized solids beds in accordance with theinvention will vary widely as a function mainly of the character of thesolids involved, particularly their specific gravity, and the characterand velocity of the gases used for fluidization and/ or reaction. It maybe stated, however, quite generally, that proper fluidization may beobtained when the proportion of solid particles fully entrainable in thefluidizing gas at the prevailing superficial gas velocity amounts toabout 30% to 50% by Weight of the fluidized bed, the proportion ofparticles having a strong settling tendency at the prevailingsuperficial gas velocity amounts to about 0% to 30% by weight of thefluidized bed, and the proportion of particles of intermediate sizeamounts to about to 70% by weight of the'fluidized bed. "The supply ofsolids fractions of extremely large or'e'X- itremely small particlesize, in the manner. described above, should be so controlled that theparticle size distribution throughout the fluidized bed is maintainedwithin these ranges.

Having set forth its objects and general nature, the invention will bebest understood from the more detailed description hereinafter in whichreference will be made to the accompanying drawing which is aschematical illustration of a system suitable for carrying out apreferred embodiment of the invention.

Referring now in detail to the drawing, the system illustratedthereinessentially comprises solids feeding equipment I, 5, and a conventionalfluid-solids reactor 30, the functions and cooperation of which will bepresently explained using the carbonization of subdivided coal as anexample. However, it should be understood that other subdivided solidsmay be treated ina substantially analogous manner.

In operation, feed hopper l contains a coal charge which may besubdivided waste coal having a particle size of less than of an inch.Large'amounts of coal waste of this character are obtained in'theconventional processing of coal at the mines. These coal wastes may havea particle size distribution about as follows:

Per cent to 4 mesh ,4 to 14 mesh 45 14 to 48 mesh 19 48 to 100 mesh 4Minus 100 mesh 7 A portion of about 20% to 60% of the total coal feeddesired to be supplied to reactor may be withdrawn from feed hopper land passed by any suitable conveying means, such as a screw conveyor,lock hopper, or. a standpipe 3, provided with aeration taps 1 and slidevalve 9 to reactor 30. If desired, the coal in hopper l-maybe preheatedwith gases from the process supp-lied through line H, to temperatures ofabout 200 to 600 F. which lie below the carbonization, plastic andignition temperatures of the coal. If no coal preheating is desired, vafluidizing gas, such as steam,- flue gas, air, etc., may be introducedthrough line 2 to facilitate the flow of the coal particles. A similarfluidizing gas may be injected, in small amounts, through taps. intostandpipe 3 to maintain the fluid character of the solids columntherein.

The fluidized coal is forced under the pseudohydrostatic pressure ofstandpipe 3, at a rate controlled by slide valve 9, into reactor 30wherein it forms above distributing grid [3 a dense turbulent mass ofcoal particles fluidized by the volatile carbonization products and agas injected through line I5 below grid l3. Superficial gas velocitiesof about 0.3 to 4 feet per second within reactor 30 are generallysuitable for this purpose. The carbonization temperature in reactor 30may be selected exclusively with a view. to the type and quantity ofvolatile carbonization products desired and may vary within the widelimits of about 800 to 2000 F. The lower temperatures within said rangeare conducive to the formation of relatively large quantities of lowtemperature tar and light oils while at the higher temperatures morecoal gas and hydrogenation products are formed.

The heat required to maintain the desired carbonization temperature maybe supplied in any conventional manner, for instance indirectly or assensible heat of the gas introduced through line l5, or by an exothermicreaction within reactor 30, such as a limited combustion of coalconstituents, or by the circulation of externally heated. char.superheated steam, hot flue or product gases, or the like are preferredheating gases in the case of low temperature carbonization. When thecarbonization is conducted above temperatures of about 1000 F., airand/or oxygen preheated to about 600 to 800 F. may be used in amounts.sufiicient to generate, by combustion, the heat required forcarbonization. About 0.3 to 1.0 pound of air per pound of coal isnormally adequate for this purpose, the exact proportion depending onthe character of the coal, the degree of preheat and the temperaturedesired.

Volatile carbonization products are Withdrawn overhead from level L30and passed through a conventional gas-solids separator 18 provided witha solids return line 20 leading, in accordance with the invention, to alower portion of the fluidized bed within reactor 30. In order to reduceentrainment of solid particles in the product gases and vapors to adeirable minimum, the top section of reactor 30 may be of enlargedcross-section as indicated at 32 so as to bring about a significantreduction in superficial gas velocity. However, entrainment of coalfines may not be completely avoided in this manner. A substantialproportion of the coal fines entrained inthe volatile carbonizationproducts are separated in separator 18 and returned through line 20 tothe bottom portion of reactor 30 to aid in the maintenance of a properparticle size distribution in accordance with the invention. Separatorl8 may also be arranged down-stream of some conventional cooling meansoutside reactor 30, if the high temperatures of reactor 30 make thisappear more advisable. Vaporous and gaseous carbonization products, nowsubstantially free of entrained coal particles, may be removed through.line 22 and passed to a conventional product recovery system (notshown). Substantially dry coke may be Withdrawn downwardly fromcarbonizer 30 through a withdrawal well 24 and line 26 for any desireduse.

At the conditions of temperature and gas velocity specified above andwhen using a large diameter, relatively shallow fluidized bed, sayhaving a depth equal to its diameter, or less, a coal particle sizedistribution suitable for proper fluidization within reactor 30 may, forexample, be about as follows:

Weight per cent 1.5 to 5 millimeters 0.1 to 1 0.5 to 1.5 millimeters 20to 25 200 to5 00 microns 50 to 60 to 200 microns 5 to 15 50 to 100microns 5 to 10 50 microns 2 to 5 er portion of the bed depending onthegasve locity employed. This classification leads to seri-. ousfluidization troubles resulting in irregularities of the temperaturethroughout the bed and the treating intensity within different sectionsof the bed. A considerable improvement is afforded by the recirculationof coal fines of less than about 200 microns size entrained in theproduct vapors and gases and separated in separator 18, to a lowerportion of the fluidized bed through line 20. The amount of coal finesso recirculated may be about 100 to 10,000 weight percent of the totalcoal fed to reactor 30, depending on the fines concentration and feedrate of the original coal feed.

The higher rates of solids flow through the cyclone and down to thebottom of the bed may be facilitated by extending the inlet to thecyclone, which may be a pipe [9, downwardly to within a short distance,say about 2 to 6 feet, from the top of the bed. The reasons for thiseilect are twofold. It is known that the efficiency of cycloneseparators increases to a certain extent as the solids load of thecyclone increases from very low levels. In addition, it has been foundthat the concentration of solids entrainment per cubic foot of gasdecreases as the gas moves away from the upper level of the fluidizedbed. For example, when using a solid material having a density of about1.0 at a superficial linear gas velocity of about 1.4 feet per second,the solids entrainment of the gas at a distance of 1 foot above thelevel may be about 0.1 pound per cubic foot while at feet above thelevel it may drop to about 0.003 pound per cubic foot.

However, there may still remain a classification of relativelylar'ge andrelatively small particles, which are not removed with the carbonizationproducts or recycled to the lower portions of the fluidized bed throughline 20. In order to eliminate fluidization troubles which may resultfrom this further classification, the invention provides for a separatefeed of relatively coarse and relatively fine coal particles to thefluidized bed in opposite directions. For this purpose, a proportion ofabout to 80% of the total coal charge to be supplied to reactor 30 iswithdrawn from feed hopper I and passed through line 4 to a conventionalclassification means, such as an elutriation system 5 wherein the coalmay be classified into two or more fractions of different averageparticle size. An elutriation gas may be supplied to the bottom ofelutriator 5 through line 6. Other conventional classification means,such as suitable sieving means, may be used.

A coal fraction comprising predominantly particles having a particlesize smaller than 200 microns may be'taken overhead from elutriator 5and passed through line 8 to a bottom portion of reactor 30, if desiredvia fluidizing gas feed line [5 and grid [3. In continuous operation,this fraction of coal particles may amount to about 5% to by weight ofthe coal supplied to elutriator 5.

Another coal fraction comprising predominantly coal particles largerthan 0.5 millimeter may be withdrawn from the bottom of elutriator 5 andpassed through line 10 to the top of reactor 30. This fraction mayamount to about 50% to 95% by weight of the coal charged to elutriator5. If desired, a third fraction of intermediate particle size may bewithdrawn from elutriator 5 through line l2 to be united with the coalcharged through line I.

The product drawn oil through line 28 may be classified in a suitableconventional device such as an elutriator or' sieve 40 and at least aportion of the fines returned through lines 42 and 8 to the fluid bed,thereby keeping the concentration of the fines in the reactor at a highlevel.

This high concentration of fines greatly improves the fluidity of thebed. The retention of the fines within the system in this manner causesthe particle size distribution within the reactor to have littleresemblance to the particle size distribution of the feed. In certaincases it may also be desirable to return atleast a portion'of the coarsesolid product separated in classifier 40, through lines 44 and ID toreactor 30.

It will be readily appreciated from the above description of the drawingthat the recirculation of coal fines and particularly the split feed ofcoal fractions of widely differing average particle size, in accordancewith. the present invention, counteract efficiently the naturalclassification tendencies of the fluidized .bed and permit themaintenance of proper fluidization conditions without careful controland readjustment of the superficial gas velocity.

While reactor ,30 and its operation have been described with referenceto the carbonization of coal, it will be understood that othercarbonizable solids, such as oil shale or cellulosic materials, may betreated substantially as described. The system may also be used for thegasification of carbonaceous solids with gasifying media, such as steamand/or carbon dioxide, by suppling the gasifying medium through line [5and maintaining a gasification temperature of, say, about 1500 to 2000F. within reactor 30. The invention, as described with reference to thedrawing, may be applied to other processes involving the use offluidized solids, such as catalytic reactions, in a generally analogousmanner, catalyst or other solids used in the process being supplied insuitable particle size ranges through lines 8, 9, and/or ID as requiredby the classification tendency of the solids mass maintained withinreactor 30.

While the foregoing description and exemplary operations have served toillustrate specific applications and results of the invention, othermodifications obvious to those skilled in the art are within the scopeof the invention. Only such limitations should be imposed on theinvention as are indicated in the appended claims.

I claim:

1. In the method of carbonizing subdivided carbonaceous solids in theform of a dense turbulent mass fluidized by an upwardly flowing gas toform a well defined upper interface and comprising component particlesreadily entrainable in said gas at the fluidization conditions and othercomponent particles tending to settle out of the fluidized mass at thefluidization conditions, the improvement which comprise supplyingreadily entrainable particles to a lower portion of said mass, supplyingparticles having a settling tendency to an upper portion of said mass soas to maintain a substantially uniform particle size distributionthroughout said mass, withdrawing solids having said particle sizedistribution from a lower portion of said mass, separating entrainableparticles from said withdrawn solids and returning at least a portion ofsaid separated particles to a lower portion of said mass.

2. The method of claim 1 in which said mass contains about 30-50% byweight of said readily entrainable particles, not more than about 30% byweight of said particles having a settling tendency and about 20-70% byweight of particles of intermediate buoyancy.

3. The method of claim 1 in which readily entrainable particlesentrained in said gas leaving said bed are separated from said gas andsupplied to a lower portion of said mass.

4. The method of claim 3 in which the solids concentration of said gasfrom which said entrained particles are separated is that prevailingabove said interface in close proximity to said bed.

5. In the method of carbonizing subdivided carbonaceous solids in theform of a dense turbulent mass fluidized by an upwardly flowing gas toform a well defined upper interface and comprising component particlesreadily entrainable.

in said gas at the fiuidization conditions and other component particlestending to settle out of the fluidized mass at the fiuidizationconditions, the improvement which comprises supplying readilyentrainable particles to a lower portion of said mass, supplyingparticles having a settling tendency to an upper portion of said mass soas to maintain a substantially uniform particle size distributionthroughout said mass, withdrawing solids having said particle sizedistribution from a lower portion of said mass, separating particleshaving a settling tendency from said withdrawn solids and returning atleast a portion of said separated solids to an upper portion of saidmass.

6. The method of claim 5, in which said mass contains about 30-50% byweight of said readily entrainable' particles, not more than about 30%by weight of said particles having a settling tendency and about 20-30%by weight of particles of intermediate buoyancy.

7. The method of claim 5 in which readily entrainable particlesentrained in said gas leaving said bed are separated from said gas andsupplied to a lower portion of said mass.

8. The method of claim 7 in which the solids concentration of said gasfrom which said entrained particles are separated is that prevailingabove said interface in close proximity to said bed.

9. In the method of carbonizing a mass of subdivided carbonaceous solidscomposed of particles spreading over a wide range of particle size inthe form of a dense turbulent bed of subdivided solids fluidized by anupwardly flowing gas to form a well defined upper interface, theimprovement which comprises withdrawing from said bed solids having theaverage particle size distribution of said bed, separating from saidwithdrawn solids a solids fraction consisting predominantly ofparticleslarge enough to tend to concentrate in the bottom of said bed at theprevailing fiuidization conditions, separating from said withdrawnsolids a solids fraction consisting predominantly of particles smallenough to tend to concentrate in the top of said bed at the prevailingfluidization conditions, feeding said first-named fraction to an upperportion of said bed and feeding said secondnamed fraction to a lowerportion of said bed so as to maintain a substantially uniform particlesize distribution throughout said bed.

10. The method of claim 9 in which said fraction of relatively largeparticle size amounts to not more than about 30% by weight and saidfraction of relatively small particle size to about 30-50% by weight, ofsaid bed.

11. The process of claim 10 in which said bed contains about 20-70% byweight of solids of an intermediate particle size.

GEORGE L. MATHESON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

1. IN THE METHOD OF CARBONIZING SUBDIVIDED CARBONACEOUS SOLIDS IN THEFORM OF A DENSE TURBULENT MASS FLUIDIZED BY AN UPWARDLY FLOWING GAS TOFORM A WELL DEFINED UPPER INTERFACE AND COMPRISING COMPONENT PARTICLESREADILY ENTRAINABLE IN SAID GAS AT THE FLUIDIZATION CONDITIONS AND OTHERCOMPONENT PARTICLES TENDING TO SETTLE OUT OF THE FLUIDIZED MASS AT THEFLUIDIZATION CONDITIONS, THE IMPROVEMENT WHICH COMPRISES SUPPLYINGREADILY ENTRAINABLE PARTICLES TO A LOWER PORTION OF SAID MASS, SUPPLYINGPARTICLES HAVING A SETTLING TENDENCY TO AN UPPER PORTION OF SAID MASS SOAS TO MAINTAIN A SUBSTANTIALLY UNIFORM PARTICLE SIZE DISTRIBUTIONTHROUGHOUT SAID MASS, WITHDRAWING SOLIDS HAVING SAID PARTICLE SIZEDISTRIBUTION FROM A LOWER PORTION OF SAID MASS, SEPARATING ENTRAINABLEPARTICLES FROM SAID WITHDRAWN SOLIDS AND RETURNING AT LEAST A PORTION OFSAID SEPARATED PARTICLES TO A LOWER PORTION OF SAID MASS.